Lightly crosslinked linear olefinic polymer foam blends and process for making

ABSTRACT

An expandable blend of a linear olefinic polymer and a crosslinkable polymer for production of lightly crosslinked foamed polymers and a process for the preparation thereof is provided. The presence of a crosslinkable polymer component in the blend increases the melt strength of the linear olefinic component so that conventional melt processing techniques may be utilized to produce foamed polymers. Both open and closed cell foams may be produced.

This is a divisional of U.S. application Ser. No. 725,036, filed Apr.19, 1985.

This invention relates to polymer foam materials made from blends of alinear olefinic polymer and a crosslinkable polymer.

It is known to make low density ethylenic polymer resin foams by theprocess of extrusion foaming, wherein a normally solid thermoplasticethylenic polymer resin such as a low density branched ethylenic polymerresin, for example, low density branched polyethylene, is melt processedand mixed under pressure with a volatile blowing agent to form aflowable gel. The gel is then passed through a shaping orifice or dieopening into a zone of lower pressure. As the pressure is lowered, thevolatile constituent of the gel vaporizes, forming cells in the resinstructure which cools to a cellular foam material.

Linear olefinic polymers, such as linear low density polyethylene, haveseveral properties which make their use in foams desirable. For example,a linear low density polyethylene has a higher modulus of elasticity,greater toughness, higher heat distortion temperature, and lowerpermeability to blowing agents than branched ethylenic polymers.However, previous attempts to produce low density foams of linearolefins by extrusion processes have been largely unsuccessful. Linearolefins, when foamed by an extrusion process, suffer from small foamcross-section, flow instability, and a narrow range of foamingtemperatures. Indeed, under some conditions, linear olefins will notfoam.

The exact cause of problems of foaming linear polyolefins is not fullyknown. However, it is generally believed that poor melt strengthtogether with a sharp change in melt viscosity near the transitiontemperature makes extrusion foaming of linear olefins difficult. Withthese properties, it is difficult to control bubble expansion duringextrusion and under conditions of heat and high shear stresses.

It is also known that relatively lightly to moderately crosslinkedthermoplastic polymers have melt properties that are suitable for foamexpansion. However, such crosslinked polymers are difficult to processon conventional melt processing apparatus such as extruders because offlow instability. At a result, most research work has been directedtoward production of crosslinked polymer compositions expandable duringpost-extrusion secondary foaming. Recently, however, advances have beenmade in overcoming some of the problems involved.

For example, Corbett, U.S. Pat. No. 4,454,086 (assigned to the assigneeof the present invention), discloses making crosslinked styrene polymerfoams by an extrusion process. Corbett teaches that a styrene acrylicacid (SAA) copolymer may be lightly crosslinked in a foam extrusion linewith a multi-functional epoxy resin. Additionally, in my owncommonly-assigned copending applications entitled, "Alcohol Control ofLightly Crosslinked Foamed Polymer Production", Ser. No. 672,101, filedNov. 16, 1984 and "Lightly Crosslinked Linear Olefinic Polymer Foams andProcess for Making", Ser. No. 672,001, filed Nov. 16, 1984, there aredisclosed expandable ethylenic and styrenic polymer compositions andcrosslinking agents for controllably crosslinking the polymers prior toextrusion foaming.

Other efforts to produce foams from linear polyolefins have centeredaround blending a linear polyolefin with another olefin polymer havinggood extrusion foamability. For example, Park et al, U.S. Pat. No.4,226,946, blended a linear polyethylene with a low density branchedpolyethylene to improve extrusion foamability of the linearpolyethylene. Watanabe et al, U.S. Pat. No. 4,102,829, blended anionomer resin with a linear polyethylene for the same purpose.Additionally, in my own commonly-assigned copending application entitled"Foams of Ionically Associated Blends of Styrenic and EthylenicPolymers", Ser. No. 653,420, filed Sept. 24, 1984, there is disclosed anionically associated blend of salts of styrene/acrylic acid andethylene/acrylic acid copolymers to produce a foam.

SUMMARY OF THE INVENTION

The present invention provides a polymeric foam made from a blend of alinear olefin, such as linear low density polyethylene, and acrosslinkable polymer having one or more reactive carboxylic acid,amide, amine, or hydroxyl functional groups. The presence of acrosslinkable polymer in the blend improves the melt strength of thelinear polyethylene so that a foam can be produced using standard meltprocessing apparatus. The blend also contains a volatile blowing agentand a crosslinking agent. Optionally, the blend also includes asufficient amount of an alcohol, such as an aliphatic alcohol havingfrom 1 to 4 carbon atoms, to control and/or delay the reversiblegas-yielding crosslinking reaction. The foam material produced exhibitsthe desirable properties of a linear olefin such as higher strengths andhigher heat distortion temperatures than corresponding branched-chainolefin foams of equivalent densities.

The linear olefin and crosslinkable polymer blend comprise from about 5to 95% by weight of a linear olefin such as linear low densitypolyethylene and from about 95 to 5% by weight of a crosslinkablepolymer such as copolymers of (a) an ethylenic or other olefinic monomerand (b) ethylenically unsaturated comonomers such as carboxylic acids,alcohols or other monomers having reactive hydroxyl groups, amines, andamides. Of course, other linear olefinic polymers such as linear highdensity polyethylene, polypropylene, and polystyrene may be utilized asthe linear olefinic component of the blend. The blowing agent may beselected from conventional volatile blowing agents such as afluorocarbon, chlorocarbon, and fluorochlorocarbon compounds. Suchblowing agents may also include other known hydrocarbons and alcohols.

The crosslinking agents found to be useful in the practice of thepresent invention include epoxy and aminofunctional silanes,organofunctional alkoxysilanes, and multiepoxyfunctional resins.Additionally, titanate compounds such as titanium alkoxides and othertitanate coupling agents as well as amino-functional crosslinking agentsare also useful.

In processing the foam, the blend of linear low density polyethylene andcrosslinkable polymer is melt processed in a conventional manner byfeeding, melting, and metering it in a conventional melt processingapparatus such as an extruder. The volatile blowing agent andcrosslinking agent, and optionally a sufficient amount of an alcohol,are mixed into the blend under pressure to form a flowable gel oradmixture.

As the flowable gel or admixture is extruded through the die opening inthe extruder to a zone of lower pressure, the volatile blowing agentactivates to expand the polymer blend to a foam structure, which islightly crosslinked to itself. Foams having densities of between 0.6 and15.0 pounds per cubic foot (pcf) (0.01 to 0.24 gm/cc) may be produced.Foams having open or closed cell structures may be produced by varyingreaction conditions and/or the starting polymer blend.

The polymer blend has improved melt strength, and the resulting polymerfoams have good dimensional stability and higher heat distortiontemperatures than comparable branched-chain olefin foams. The foams ofthe present invention are useful as sound insulation panels, structuralfoams, cushioning packaging material, thermocollapse resistant films,and crosslinked jackets for cables and wires. Additionally, open cellstructure foams of the present invention may find use as soundinsulation materials as well as filter materials for the chemical,pharmaceutical, and tobacco industries.

Accordingly, it is an object of the present invention to provide blendsof linear olefinic polymer and crosslinkable polymer compositions, andprocesses which can be utilized to expand such blends to useful foamedmaterials, using conventional melt processing techniques. This, andother objects and advantages of the present invention, will becomeapparent from the following detailed description, the accompanyingdrawings, and the appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a graph of the effect of an epoxy functional silanecrosslinking agent on extruder discharge pressure for a 50/50 blend byweight of a linear low density polyethylene (melt index of 2.0 anddensity of 0.926 gm/cc) and a copolymer of ethylene and acrylic acid;and

FIG. 2 is a graph of aging temperature versus percent volume retentionfor a comparative branched-chain olefin and linear olefin blend foam.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The process and composition of the present invention permit theproduction of low density foam material by an extrusion process from alinear low density olefinic polymer by blending with it a crosslinkablepolymer. This provides a foam having the desirable properties of linearlow density olefins, such as higher temperature resistance and higherheat distortion temperatures than branched-chain olefin foams ofequivalent density. The present invention is also unique in that byvarying the process conditions and/or starting polymer blend, thestructure of the foam produced can be open cell or have substantialnumbers of closed cells.

The polymer blend of the present composition comprises from about 5 to95% by weight of a linear olefin such as linear low density polyethylene(LLDPE) and from about 95 to 5% by weight of a crosslinkable polymer. Amost preferred range in the practice of this invention is from 40-70% byweight linear olefin to 60-30% by weight of the crosslinkable polymer.The crosslinkable polymer may be a copolymer of (a) an ethylenic orother olefinic monomer with (b) an ethylenically unsaturated monomerhaving a carboxylic acid functional group, a hydroxyl group, or an amineor amide group. The monomers may be combined in the copolymer in anyway, e.g., as random copolymer, as block or sequential copolymers, or asgraft copolymers. Materials of these kinds and methods of making themare readily known in the art. Most preferred as the crosslinkablepolymer component of the blend are copolymers of ethylene and acrylicacid. These crosslinkable polymers improve the melt strength of thelinear low density olefinic component of the blend and permit successfulfoam extrusion on conventional melt processing apparatus.

The blowing agents useful in the practice of the present invention arewell known and may comprise solids which decompose into gaseous productsat extrusion temperatures or volatile liquids. A preferred class ofblowing agents is the group of halogenated hydrocarbon compounds havingfrom 1 to 4 carbon atoms. Dichlorodifluoromethane (FC-12),trichloromonofluoromethane (FC-11), and 1,2 dichlorotetrafluoroethane(FC-114) are most preferred. When these halogenated hydrocarboncompounds are used as the blowing agent, there can be from about 0.013to 0.50 gram mole, and preferably about 0.040 to 0.30 gram mole of suchblowing agent per 100 grams of polymer resin blend in the flowable gelor admixture. Mixtures of such blowing agents may also be used.

The blowing agent is compounded into the flowable gel in the meltprocessing apparatus in proportions to make the desired degree ofexpansion in the resulting foamed cellular product to make productshaving foamed densities down to about 0.6 pounds per cubic foot (pcf)(0.01 gm/cc). Depending on the amount of blowing agent added, theresulting foamed materials may have densities from about 0.6 to 15.0 pcf(0.01 to 0.24 gm/cc).

Crosslinking agents useful in the practice of the present inventioninclude epoxy and amino functional silanes, organofunctional alkoxysilanes, multiepoxyfunctional resins, titanates, and amines. Thesecrosslinking agents react with the crosslinkable polymer component ofthe blend to form lightly crosslinked bonds. This light crosslinking ofthe polymer blend improves melt strength and permits successful foamextrusion on conventional melt processing equipment.

Some of the crosslinking agents used in the practice of the presentinvention form crosslinking bonds by a reaction which releases analcohol. For example, alkoxy functional silane crosslinking agents grafton ethylenic polymers having carboxylic acid groups to form acyloxysilane linkages with the release of alcohol. Likewise, amino andepoxyfunctional silanes graft on polymers having carboxylic acid oranhydride groups, again with the release of alcohol. The presence ofalcohol in the foam extrusion line may be used to control thecrosslinking reaction, effectively delaying crosslinking until thepolymer blends exits the extrusion die.

The preferred silane crosslinking agents are organofunctional silanes ofthe general formula RR'SiY₂, where R represents an epoxy or aminefunctional radical attached to silicon through a silicon carbon bond andcomposed of carbon, hydrogen, and optionally oxygen or nitrogen, Yrepresents a hydrolyzable organic radical, and R' represents ahydrocarbon radical of Y. Alternatively, the silane may be an alkoxysilane of the general formula R_(a) Si(OR')_(b), where "a" is 1 or 2 and"b" is 2 or 3, R is methyl or an organoreactive alkyl group, and OR' isa hydrolyzable alkoxy group.

Preferred multiepoxyfunctional resins include an epoxy novolac resin,D.E.N. 431, commercially available from the Dow Chemical Company. Suchmultiepoxyfunctional resins have multiple epoxy functional reactionsites which will react with carboxylic acid functional groups on thecrosslinkable polymer.

The preferred titanate crosslinking agents are titanium alkoxides of thegeneral formula Ti(OR)₄, where R is an alkyl group of from 1 to 18carbon atoms, or titanium coupling agents of the general formula(RO)_(m) Ti(O--X--R₂ --Y)_(n), where R is an alkyl group, X is carbonyl,R₂ is a long chain of carbon atoms, Y is a reactive double bond oramino, and "m" and "n" are integers which total 4. The most preferredtitanate coupling agents are titanium isopropoxide and tetramethyltitanate. These titanate crosslinking agents react with carboxylic acidor hydroxyl functional groups on the crosslinkable polymer releasingalcohols.

Preferred amino crosslinking agents are hexamethoxymethylmelamine (HMMM)and alkylated glycolurilformaldehyde resins. These amino crosslinkingagents react with hydroxy, carboxylic acid, or amide functional groupson the crosslinkable polymer.

The crosslinking agents of the present invention are added to thepolymer gel blend with the blowing agent and react with thecrosslinkable polymer component of the blend. This crosslinkingincreases the melt tension and melt viscosity of the gel, whilepermitting the polymer to remain flowable. As explained above, some ofthe crosslinking agents used in this invention form alcohols as theresult of the crosslinking reaction and act to limit the degree ofcrosslinking. However, in such instances the crosslinking reactionproceeds during foam expansion at the exit of the die as the alcoholdiffuses into the gaseous phase with the volatile blowing agent.Optionally, an alcohol may be added to the blowing agent to controlfurther the crosslinking reaction. Preferably, such alcohols are of lowmolecular weight having from 1 to 4 carbon atoms such as methanol,ethanol, isopropanol and butanol.

In accordance with the process of the present invention, foams of ablend of a linear olefinic polymer with a crosslinkable polymer may bemade on conventional melt processing apparatus such as by continuousextrusion from a screw-type extruder. Such an extruder typicallycomprises a series of sequential zones including a feed zone,compression and melt zone, metering zone, and mixing zone. The barrel ofthe extruder may be provided with conventional electric heaters forzoned temperature control.

An inlet is provided for adding a mixture of fluid blowing agent andcrosslinking agent under pressure to the polymer blend in the extruderbarrel between the metering and mixing zones. Crosslinking agent ispumped, in a controllable manner, into the stream of fluid blowing agentupstream of the injection nozzle. The blowing agent and crosslinkingagent are compounded into the starting polymer in a conventional mannerto form a flowable gel or admixture, preferably in a continuous manner.Thus, the polymer blend, blowing agent, and crosslinking agent may becombined in the mixing zone of an extruder using heat to plastify thepolymer resin, pressure to maintain the blowing agent in a liquid state,and mechanical working to obtain thorough mixing.

The discharge end of the mixing zone of the extruder is connected,through a cooling and temperature control zone, to a die orifice. Thehot polymer gel is cooled and then passed through the die orifice into azone of lower pressure (e.g. normal ambient air atmosphere) where theblowing agent is activated and the polymer gel expands to a lowerdensity, cellular mass. As the foamed extrusion forms, it is conductedaway from the die and allowed to cool and harden.

In practice, the temperature of the feed zone is maintained at 180°±20°C., the temperature of the melting, metering, and mixing zones ismaintained at 210°±20° C., and the temperature in the cooling andtemperature control zone is maintained at 120°±20° C. The temperature ofthe polymer gel as it expands through the die orifice is preferably justabove the temperature at which solid polymer would crystallize out ofthe gel and will vary depending upon the particular polymer blendutilized. For example, the crosslinkable polymer component of the blendmay be selected so that it has a lower melting point than the linearolefinic polymer. Operation at near the freezing point of the linearolefin will still be above the melting point of the crosslinkablepolymer, resulting in the polymer gel remaining flowable.

The resulting linear olefinic polymer blend foams are flexible tobending and shaping. The foams have excellent dimensional stability andhigher compressive strengths and heat distortion temperatures thanbranched low density polyethylene foams having an equivalent foamdensity.

A unique aspect of the present invention is the ability to control thefoaming process parameters so that excellent quality foams having eitheropen or closed cell structures may be produced from the starting polymerblends. Ordinarily, it is quite difficult to produce foams having anopen cell structure by an extrusion process. Open cell foams have uniqueproperties which render them useful as sound insulation materials andfilter media for a variety of end uses. Of course, foams with asubstantially closed cell structure have other known uses including useas cushion packaging material.

The percentage of closed cells formed in the final foams is controlledby varying the operating temperature of the extrusion process. Excellentquality open cell foams are produced by operating the cooling zone ofthe extruder at a temperature slightly (e.g., 2°-4° C.) above thefreezing point of the linear olefinic component of the polymer blend.Lowering the temperature in the cooling zone toward the freezing pointof the linear olefinic component produces foams having a higher degreeof closed cells.

As is conventional, finely divided solid materials such as talc, calciumsilicate, zinc stearate, and the like can advantageously be incorporatedwith the polymer gel prior to expansion. Such finely divided materialsaid in controlling the size of the cells and may be employed in amountsup to five percent by weight of the polymer. Numerous fillers, pigments,lubricants, antioxidants and the like well known in the art can also beincorporated as desired.

The specific working examples that follow are intended to illustrate theinvention but are not to be taken as limiting the scope thereof. In theexamples, parts and percentages are by weight unless otherwise specifiedor required by the context.

EXAMPLE I

The apparatus used in this example is a 11/4" (31.75 mm) screw typeextruder having two additional zones for mixing and cooling at the endof usual sequential zones for feeding, melting and metering. An openingfor blowing agent injection is provided on the extruder barrel betweenthe metering and mixing zones. A small syringe-type pump is connected tothe blowing agent stream for additive injections. At the end of coolingzone, there is attached a die orifice having an opening of rectangularshape. The height of the opening, called die gap hereinafter, isadjustable while its width is fixed at 0.25" (6.35 mm).

In this example, it is shown that a high temperature-resistant linearpolyolefin foam blend can be produced from a blend of a linear lowdensity polyethylene and an ethylene/acrylic acid copolymer. Thus, thepolymer used in this example is a 50/50 by weight blend of a linear lowdensity polyethylene, Dowlex 2032 (2.0 melt index, 0.926 g/cc density) agranular copolymer of acrylic acid with ethylene (2.0 melt index and6.5% acrylic acid). Throughout the tests in this example, a small amountof talcum powder (0.2-0.7 pph) was added for cell size control.Optionally, a small amount (0.05 pph) of magnesium oxide was put in tocatalyze the epoxy/acid reaction.

An 80/20 by weight mixture of FC-12/FC-11(dichlorodifluoromethane/trichloromonofluoromethane) was employed as theblowing agent in the tests of this example. Methanol was fed in theextruder in a mixture with the blowing agent in tests designed to seeits effect as the reaction-delaying agent. Formulations in the test ofthis example are presented in Table A.

The temperatures maintained at extruder zones were approximately 120° C.at feeding zone, 190° C. at melting and metering zone and also at mixingzone. The temperature of cooling zone was maintained so that thetemperature of the polymer/blowing agent mixture could reach an optimumuniform temperature for foaming which was in the range of 115°-119° C.as shown in Table A. The die gap was fixed at 0.120" (3.0 mm) throughoutthe tests. In some tests, foam dimensional stability at ambienttemperature was followed with specimens cut to about 7 inches (17.8 cm)in length. The foam cross sectional area varied with the formulation butmost had width in the range of 1.0-1.5" (2.54-3.8 cm) and thickness0.7-1.0" (1.78-2.54 cm).

Table I shows the results of the tests. With no crosslinking, theresultant foam totally collapsed. Addition of epoxy functional silanemade the foam rise. Magnesium oxide assisted the crosslinking reaction.At an epoxy silane level of 0.3 pph with magnesium oxide incorporated, areasonably good looking foam of substantially open cell structure wasobtained. Dimensional stability of the open cell foam was good. Furtherincreases in the amount of crosslinking agent, however, made the foamstrand unstable. That is, the foam strand became wobbly and in extremecases fractured. Note that addition of crosslinking agent raised theextruder discharge pressure as much as 600 psi.

Addition of methanol dramatically reduced the line pressure as seen inTests 9 and 10. Also, addition of methanol cured the flow instability ofthe extruder and further resulted in good foams having substantiallyclosed-cell structure. FIG. 1 illustrates the effects of the amount ofcrosslinking agent and the presence of magnesium oxide on the extruderdischarge pressure.

                                      TABLE I                                     __________________________________________________________________________                                       FOAM         DIMEN-                            BA   MgO  Z-6040                                                                             MeOH GEL  EXTRUD.                                                                             DEN- CELL                                                                              OPEN                                                                              SIONAL FLOW                   TEST                                                                              LEVEL                                                                              LEVEL                                                                              LEVEL                                                                              LEVEL                                                                              TEMP.                                                                              PRES. SITY SIZE                                                                              CELL                                                                              STABILITY                                                                            STABILITY              NO. (1)  (2)  (3)  (4)  (5)  (6)   (7)  (8) (9) (10)   (11)                   __________________________________________________________________________    1   22.1 0    0    0    118.5                                                                              69.5  --   --  --  --     Foam Collapses         2   21.0 0    0.2  0    117  80.2  45.5 1.47                                                                              93  --     Foam Rises             3   21.6 0.05 0.2  0    117.5                                                                              94.2  30.3 2.03                                                                              92  --     Foam Improves          4   21.0 0    0.3  0    117.2                                                                              94.2  39.9 1.62                                                                              93  --     Foam Improves          5   21.1 0.05 0.3  0    117.5                                                                              109.0 27.4 1.47                                                                              95  101    Looks Good             6   20.8 0    0.4  0    116  106.2 27.1 1.62                                                                              91  97     Unstable               7   21.6 0.05 0.4  0    116.5                                                                              106.9 27.1 1.62                                                                              84  --     Unstable               8   22.7 0    0.5  0    116  108.3 24.2 1.80                                                                              84  99     Unstable               9   17.8 0    0.5  1.98 115  62.6  25.9 1.35                                                                              59  --     Verge of                                                                      Prefoaming             10  18.5 0.05 0.5  2.06 116.5                                                                              70.3  24.5 1.62                                                                              43  55     Looks                  __________________________________________________________________________                                                           Good                    (1) parts of 80/20 by weight mixture of FC12/FC-11 mixed in per hundred       parts of polymer                                                              (2) parts of magnesium oxide mixed in per hundred parts of polymer            (3) parts of Dow Corning epoxy functional silane Z6040 mixed in per           hundred parts of polymer                                                      (4) parts of methanol mixed in per hundred parts of polymer                   (5) temperature of gel coming out of the die in degrees centigrade            (6) pressure at the extruder discharge in kilograms per square centimeter     (7) density of foam body in kilograms per cubic meter measured in about       one month                                                                     (8) cell size in millimeter in horizontal direction determined per ASTM       D3576                                                                         (9) open cell in percent determined per ASTM D2856-A                          (10) minimum or maximum volume of foam body during aging as percentage of     initial volume measured within about five minutes after extrusion             (11) appearance of foam body being extruded out of the die orifice       

The heat distortion characteristics of the blend foam produced in Test10 were tested against a foam produced from a branched-chain low densitypolyethylene having 2.3 melt index and 0.921 g/cc density. The foamswere placed in an oven for one hour at temperatures between about 100°and 130° C. The percent volume retention of the foams was measured asthe ratio of final to initial foam volume. FIG. 2 illustrates the hightemperature stability of those foams. The blend foam of Test 10 hadsuperior high temperature performance.

EXAMPLE II

The apparatus and operating procedures used for the tests in thisexample were the same as in Example I. The polymer feedstock was a 50/50by weight blend of a granular linear low density polyethylene having a1.0 melt index (ASTM D-1238-79 Condition E) and a 0.935 gm/cc densityand a granular copolymer of acrylic acid and ethylene (2.0 melt indexand 6.5% acrylic acid). A small amount of talc (0.05 pph) was added tothe blend for cell size control.

The blend was fed into the extruder at 10 pounds (4.55 kg) per hour.Dichlorodifluoromethane was used as the blowing agent and was injectedinto the extruder at a rate of about 1.72 pounds (0.78 kg) per hour. Thecrosslinking agent was hexamethoxymethylmelamine (HMMM) which wasdissolved in a 50% solution with methylene chloride. The HMMM solutionwas injected into the extruder by a syringe pump. The temperaturesmaintained in the extruders zones were as follows: 150° C. at thefeeding zone, 180° C. at the melting and metering zone, and 200° C. atthe mixing zone. The temperature of the cooling zone was adjusted sothat the polymer gel could be cooled to about 125° C. throughout thetests. The die gap was varied to test its effect on foam appearance.

                                      TABLE II                                    __________________________________________________________________________        BA   HMMM GEL  DIE                                                                              DIE EXTRUDER                                                                             FOAM FOAM FOAM  CELL                                                                              OPEN                                                                              FOAM                 TEST                                                                              LEVEL                                                                              LEVEL                                                                              TEMP.                                                                              GAP                                                                              PRES.                                                                             PRESS. THICK                                                                              WIDTH                                                                              DENSITY                                                                             SIZE                                                                              CELL                                                                              APPEAR.              NO. (1)  (2)  (3)  (4)                                                                              (5) (6)    (7)  (8)  (9)   (10)                                                                              (11)                                                                              (12)                 __________________________________________________________________________    1   17.1 --   126  0.25                                                                             24.6                                                                              63.3   1.57 2.03 290   1.35                                                                              67.6                                                                              C                                       0.23                                                                             26.0                                                                              65.0   1.57 2.03 211   1.81                                                                              71.6                                                                              C                                       0.20                                                                             28.5                                                                              67.5   1.35 2.34 167   1.81                                                                              74.1                                                                              C                                       0.18                                                                             32.3                                                                              73.8   1.19 3.05 86.8  2.03                                                                              83.7                                                                              PC                                      0.15                                                                             35.2                                                                              75.9   1.07 2.92 91.0  2.03                                                                              83.1                                                                              PC                                      0.13                                                                             40.8                                                                              80.9   1.02 3.28 55.9  1.81                                                                              86.6                                                                              PC                                      0.10                                                                             48.5                                                                              91.4   0.84 3.30 56.0  1.62                                                                              86.4                                                                              PC                   2   17.2 0.1  125  0.25                                                                             45.0                                                                              106    1.68 3.25 47.2  1.62                                                                              89.0                                                                              PC                                      0.23                                                                             47.8                                                                              112    1.73 3.48 34.7  1.47                                                                              88.9                                                                              G                                       0.22                                                                             49.2                                                                              113    1.63 3.45 34.1  1.62                                                                              90.0                                                                              G                                       0.20                                                                             52.0                                                                              114    1.60 3.48 34.4  1.47                                                                              88.4                                                                              E                                       0.19                                                                             55.5                                                                              122    1.57 3.56 31.2  1.81                                                                              87.2                                                                              E                    3   17.2 0.125                                                                              125  0.25                                                                             46.4                                                                              117    2.11 3.45 30.6  1.25                                                                              91.1                                                                              G                                       0.23                                                                             46.8                                                                              119    1.88 3.48 29.3  1.81                                                                              87.7                                                                              E                                       0.22                                                                             51.0                                                                              124    1.83 3.56 29.1  1.62                                                                              89.3                                                                              E                                       0.20                                                                             54.1                                                                              130    1.78 3.61 29.5  1.62                                                                              79.7                                                                              E                                       0.19                                                                             59.4                                                                              137    1.60 3.65 29.3  1.81                                                                              87.8                                                                              E                    __________________________________________________________________________     (1) parts of dichlorodifluoromethane mixed in per hundred parts of polyme     (2) parts of effective hexamethoxymethylmelamine mixed in per hundred         parts of polymer                                                              (3) temperature in degrees centigrade to which the gel was cooled down        prior to foam expansion                                                       (4) gap of die opening in centimeters at which samples were taken             (5), (6) pressure in kilograms per square centimeter at the die and at th     extruder discharge, respectively                                              (7), (8) thickness and width of foam body in centimeters measured within      about five minutes after extrusion                                            (9) density of foam body in kilograms per cubic meter measured in about       one month                                                                     (10) cell size in millimeter in horizontal direction determined per ASTM      D3576                                                                         (11) open cell content in percent determined per ASTM D2856-A                 (12) subjective judgment of foam appearance, C = total collapse, PC =         partial collapse, G = good, E = excellent                                

As can be seen from Table II, without the addition of a crosslinkingagent, no foams could be produced at any die gap. All foams totally orpartially collapsed under those conditions. However, the addition ofHMMM had a dramatic effect on foam appearance. At an HMMM level of 0.1pph, excellent quality foams were produced at die gaps of 0.08 inches(2.0 mm) and smaller. At somewhat higher HMMM levels, the die gap couldbe opened even wider without incurring prefoaming of the polymer gel.While all foams contained relatively high levels of open cells, the foamappearance and structure was otherwise good. Such open-celled foams areuseful in sound insulation applications. It was found that operation atslightly lower gel temperatures will reduce the open cell content of thefoam.

EXAMPLE III

The apparatus, operating procedures, polymer blend, blowing agent, andcell size control agent used in this example were the same as in ExampleII. A multiepoxyfunctional novolac resin, designated D.E.N. 431 from DowChemical Company, was employed as the crosslinking agent. The epoxyresin was dissolved in a 50% solution with methylene chloride andinjected into the extruder. A small amount (0.05 pph) of magnesium oxidewas mixed into the polymer feedstock to catalyze the crosslinkingreaction. Blowing agent was injected at a uniform rate of 1.67 pounds(0.76 kg) per hour.

As shown by Table III, the epoxy resin crosslinking agent raises the diepressure and improves foam appearance. At a level of 0.125 pph of theepoxy resin, the die was opened as wide as 0.13 inches (0.33 cm) whilestill producing a good quality foam.

                                      TABLE III                                   __________________________________________________________________________             D.E.N.                                                                   BA   431  GEL  DIE                                                                              DIE EXTRUDER                                                                             FOAM FOAM FOAM  CELL                                                                              OPEN                                                                              FOAM                 TEST                                                                              LEVEL                                                                              LEVEL                                                                              TEMP.                                                                              GAP                                                                              PRES.                                                                             PRESS. THICK                                                                              WIDTH                                                                              DENSITY                                                                             SIZE                                                                              CELL                                                                              APPEAR.              NO. (1)  (2)  (3)  (4)                                                                              (5) (6)    (7)  (8)  (9)   (10)                                                                              (11)                                                                              (12)                 __________________________________________________________________________    1   16.7 0.1  125  0.25                                                                             34.5                                                                              90.7   1.78 3.30 39.4  1.81                                                                              8.88                                                                              G                                       0.23                                                                             35.9                                                                              94.9   1.65 3.40 40.2  1.81                                                                              89.6                                                                              G                                       0.22                                                                             40.8                                                                              101    1.65 3.35 35.2  1.47                                                                              93.0                                                                              G                                       0.20                                                                             43.0                                                                              101    1.63 3.43 36.2  1.62                                                                              89.1                                                                              G                                       0.19                                                                             45.7                                                                              106    1.52 3.43 33.1  1.35                                                                              90.4                                                                              E                                       0.18                                                                             48.5                                                                              109    1.50 3.45 33.0  1.62                                                                              89.6                                                                              E                    2   16.7 0.125                                                                              125  0.33                                                                             42.2                                                                              120    2.44 3.45 35.1  2.03                                                                              88.1                                                                              E                                       0.30                                                                             43.6                                                                              123    2.39 3.40 35.7  2.03                                                                              87.5                                                                              E                                       0.28                                                                             46.4                                                                              127    2.29 3.43 34.9  1.81                                                                              87.7                                                                              E                                       0.25                                                                             48.5                                                                              127    2.26 3.40 33.8  1.62                                                                              89.7                                                                              E                                       0.23                                                                             51.3                                                                              131    2.18 3.38 32.5  2.03                                                                              87.9                                                                              E                                       0.20                                                                             55.5                                                                              137    2.11 3.30 33.5  1.81                                                                              85.7                                                                              E                    __________________________________________________________________________     (1) parts of dichlorodifluoromethane mixed in per hundred parts of polyme     (2) parts of epoxy novolac resin D.E.N. 431 produced by Dow Chemical          Company mixed in per hundred parts of polymer                                 (3) temperature in degrees centigrade to which the gel was cooled down        prior to foam expansion                                                       (4) gap of die opening in centimeters at which samples were taken             (5), (6) pressure in kilograms per square centimeter at the die and at th     extruder discharge, respectively                                              (7), (8) thickness and width of foam body in centimeters measured within      about five minutes after extrusion                                            (9) density of foam body in kilograms per cubic meter measured in about       one month                                                                     (10) cell size in millimeter in horizontal direction determined per ASTM      D3576                                                                         (11) open cell content in percent determined per ASTM D2856-A                 (12) subjective judgment of foam appearance; C = total collapse, PC =         partial collapse, G = good, E = excellent                                

EXAMPLE IV

The apparatus, operating procedures, polymer blend, and cell sizecontrol agent used in this example were the same as in Example III. Asthe crosslinking agent, the epoxyfunctional silane used in Example I wasused. A small amount (0.05 pph) of magnesium oxide was added to catalyzethe crosslinking reaction.

As shown in Table IV, the addition of 0.2 pph of the epoxyfunctionalsilane crosslinking agent provides improvement in the foamability of thepolymer blend. Under the conditions of this example, the addition of0.35 pph of the silane resulted in melt fracture (over crosslinking) ofthe polymer blend.

                                      TABLE IV                                    __________________________________________________________________________        BA   Z-6040                                                                             GEL  DIE                                                                              DIE EXTRUDER                                                                             FOAM FOAM FOAM  CELL                                                                              OPEN                                                                              FOAM                 TEST                                                                              LEVEL                                                                              LEVEL                                                                              TEMP.                                                                              GAP                                                                              PRES.                                                                             PRESS. THICK                                                                              WIDTH                                                                              DENSITY                                                                             SIZE                                                                              CELL                                                                              APPEAR.              NO. (1)  (2)  (3)  (4)                                                                              (5) (6)    (7)  (8)  (9)   (10)                                                                              (11)                                                                              (12)                 __________________________________________________________________________    1   17.3 0.2  125  0.25                                                                             38.0                                                                              103    1.83 3.23 28.5  1.47                                                                              50.1                                                                              G                                       0.23                                                                             42.2                                                                              110    1.80 3.30 26.7  1.25                                                                              91.1                                                                              E                                       0.22                                                                             45.0                                                                              112    1.78 3.33 25.8  1.47                                                                              85.1                                                                              E                                       0.20                                                                             46.4                                                                              119    1.70 3.35 26.3  1.47                                                                              74.1                                                                              E                                       0.19                                                                             48.5                                                                              120    1.63 3.35 24.8  1.81                                                                              68.4                                                                              E                                       0.18                                                                             52.0                                                                              123    1.47 3.33 23.7  1.16                                                                              59.9                                                                              E                    2   17.2 0.35 125  0.51                                                                             29.5                                                                              134    1.91 2.61 42.3  0.52                                                                              90.7                                                                              melt                                                                          fracture             __________________________________________________________________________     (1) parts of dichlorodifluoromethane mixed in per hundred parts of polyme     (2) parts of Dow Corning epoxy functional silane mixed in per hundred         parts of polymer                                                              (3) temperature in degrees centigrade to which the gel was cooled down        prior to foam expansion                                                       (4) gap of die opening in centimeters at which samples were taken             (5), (6) pressure in kilograms per square centimeter at the die and at th     extruder discharge, respectively                                              (7), (8) thickness and width of foam body in centimeters measured within      about five minutes after extrusion                                            (9) density of foam body in kilograms per cubic centimeter measured in        about one month                                                               (10) cell size in millimeter in horizontal direction determined per ASTM      D3576                                                                         (11) open cell content in percent determined per ASTM D2856-A                 (12) subjective judgment of foam appearance; C = total collapse, PC =         partial collapse, G = good, E = excellent                                

While the methods and compositions herein described constitute preferredembodiments of the invention, it is to be understood that the inventionis not limited to these precise methods and compositions, and thatchanges may be made in either without departing from the scope of theinvention, which is defined in the appended claims.

What is claimed is:
 1. An expandable polymeric composition comprising ablend of a linear olefin polymer and a crosslinkable polymer having oneor more carboxylic acid, amide, amine, or hydroxyl functional groups; acrosslinking agent selected from the group consisting ofepoxy-functional silanes, amino-functional silanes, organofunctionalalkoxy silanes, amines, multiepoxyfunctional resins, and titanates; anda volatile blowing agent.
 2. The composition of claim 1 in which saidlinear olefin polymer is low density polyethylene and is present in anamount of from 5 to 95% by weight and said crosslinkable polymer ispresent in an amount of from 95 to 5% by weight.
 3. The composition ofclaim 1 in which said crosslinkable polymer is an ethylene-acrylic acidcopolymer.
 4. The composition of claim 3 in which said crosslinkingagent is an epoxyfunctional silane.
 5. The composition of claim 3 inwhich said crosslinking agent is a multiepoxyfunctional resin.